In a liquid crystal display (LCD), a backlight control circuit is used which controls LEDs to illuminate from the back side of an LCD screen, so that a user can observe an image from the front side of the LCD screen.
In early days, LED backlight is used only in a small size screen, which does not require high backlight brightness. Therefore, the LEDs can be connected all in series or all in parallel. FIG. 1 shows a prior art circuit wherein all LEDs are connected in series. As shown in the figure, a backlight control circuit 10 comprises a voltage supply circuit 11 providing output voltage Vout to a plurality of LEDs L1-LN connected in series. A resistor R is provided on a path of the LEDs connected in series, and a voltage at a node Vsense1 is compared with a reference voltage Vref to check whether a current through the path satisfies a predetermined condition. If the current is lower than a predetermined value and the voltage at the node Vsense1 decreases, an error amplifier circuit 13 sends a signal 15 to the voltage supply circuit 11 to pull up the output voltage Vout, so that the current flowing through the LEDs increases. Moreover, to prevent the voltage supply circuit 11 from unlimitedly increasing the output voltage Vout (for example, when the error amplifier circuit 13 malfunctions, or when the path of the LEDs is open), an over voltage protection circuit 12 is provided in the backlight control circuit 10, which detects the output voltage Vout and sends a signal to stop the voltage supply circuit 11 from increasing the output voltage Vout if the output voltage Vout is excessively high. (Depending on circuit design, the voltage supply can be totally stopped, or kept at an upper limit value. The latter is more popular in a backlight control circuit.)
FIG. 2 shows a typical structure of an over voltage protection circuit 12, wherein the output voltage Vout is monitored by comparing the voltage at the node Vsense2 with a reference voltage Vovp. The result of comparison determines a signal for controlling the voltage supply circuit 11.
The above arrangement wherein all LEDs are connected in series has several drawbacks. The output voltage Vout can not be too high for cost and safety reasons; therefore, the number of LEDs that can be connected in series is limited. When the backlight brightness requirement increases for a larger size LCD screen, it becomes impossible to connect all LEDs in one path. In addition, due to series connection, if one LED shuts down, all the other LEDs are shut down; the LCD will be in complete darkness.
Referring to FIG. 3, it shows a conventional backlight control circuit with LEDs all connected in parallel. As shown in the figure, in a backlight control circuit 20, the currents passing through LEDs L1-LN are respectively controlled by the current sources CS1-CSN. The backlight control circuit 20 comprises a lowest voltage selection circuit 21 which chooses a lowest voltage value among all voltages at cathode ends of the LEDs L1-LN, and the error amplifier circuit 13 compares the lowest voltage value with a reference voltage to generate a signal controlling the voltage supply circuit 11. Thus, the output voltage Vout is under control so that all current source circuits are provided with sufficient operating voltage for normal operation, and all LEDs can illuminate normally thereby.
Similarly, the backlight control circuit 20 can further comprise an over voltage protection circuit 12 as the one described above.
The above arrangement wherein all LEDs are connected in parallel has the following drawbacks. Because the backlight control circuit 20 is an integrated circuit, the number of its pins (shown by hollow squares in the figure) is limited for cost and other reasons, which limits the number of LEDs to be connected. When the backlight brightness requirement increases for a larger size LCD screen, the number of pins becomes insufficient. In addition, if one of the LEDs shuts down, or if a pin is short to ground, the lowest voltage selection circuit 21 will select the input corresponding to the shut-down LED or the grounded pin, and the error amplifier circuit 13 will keep asking the voltage supply circuit 11 to increase the output voltage Vout. Under the circumstance, the voltage supply circuit 11 can not adjust its output voltage Vout according to normal LEDs. In the case where an over voltage protection circuit is provided, the output voltage Vout will be kept at the upper limit, causing unnecessary power consumption and reducing power efficiency. In the case where no over voltage protection circuit is provided, the integrated circuit, sometimes the LEDs as well, may be burned out. Furthermore, if the number of pins of the integrated circuit is larger than required, excess pins have to be connected to the output voltage Vout to avoid malfunction by the lowest voltage selection circuit 21, but such solution is not desired because it wastes the energy provided at the output terminal, and it causes heat and other issues.
Because the number of LEDs that are allowed to be connected all in series or all in parallel in the above conventional arrangements is limited, it naturally leads to connecting the LEDs partially in series and partially in parallel (series-parallel connection). FIG. 4 shows a prior art arrangement of such series-parallel connection in which the backlight control circuit 10 shown in FIG. 1 is employed to provide voltage to a series-parallel connection circuit of LEDs. However, it only checks the current on the path of LEDs L1-LN but does not check those on the other paths. Apparently, this is not a perfect arrangement because the LEDs in the other paths are not under control; the currents flowing through the LEDs are not accurate.
In another prior art arrangement, multiple backlight control circuits 10 are employed each of which controls one LED path so that every path is under control. Although such multiple backlight control circuits 10 can be integrated in one integrated circuit, it is obviously not cost effective.
Another prior art arrangement is shown in FIG. 5 which employs the backlight control circuit 20 shown in FIG. 3 to compose a series-parallel connection circuit for LEDs. In this arrangement, although the number of LEDs to be connected is increased, it still has the drawback that if one of the LED paths is open, the lowest voltage selection circuit 21 will force the voltage supply circuit 11 to increase the output voltage Vout unlimitedly. Moreover, if the number of pins of the integrated circuit is more than required, the excess pins becomes a hot potato; the output voltage Vout in FIG. 5 is a much higher voltage (for supplying operative voltage to multiple LEDs connected in series) than the output voltage Vout in FIG. 3, and thus if the excess pins are connected to the output voltage Vout, the device inside the current source circuit has to be a costly high voltage device. However, if the excess pins are not connected to the output voltage Vout, there is no low-cost solution to arrange those pins. As such, the circuit shown in FIG. 5 can only be applied to limited applications, and it has low immunity, that is, it is vulnerable to malfunctions and other circuit errors.